The invention relates to an MR process to locate a medical instrument with a microcoil attached thereto in the examination volume of an MR device.
The invention also relates to an MR device for performance of such a process and a computer program to implement such a process on an MR device.
In the field of interventional radiology, processes based on magnetic resonance (MR) are now becoming increasingly important. In contrast to radio diagnostics previously generally used in this field, magnetic resonance has the advantage that neither the patient nor the doctor performing the work is exposed to ionizing radiation. Also MR processes have the advantage of far better soft tissue contrast than processes of radio diagnostics.
For interventional radiological processes, the location of the interventional medical instruments used always plays a decisive role. These instruments can be for example intravascular catheters, biopsy needles, minimal invasive surgical instruments or similar. One important use of interventional radiology is angiography i.e. radiological processes to clarify the anatomical details of a patient's blood system. Angiographic MR processes are of particular interest for examining blood vessels by means of intravascular catheters which at their tips are equipped with special microcoils for the purposes of location.
U.S. Pat. No. 6,236,205 B1 discloses an MR-based process to locate a medical instrument with attached microcoil. In the previously known process, the microcoil is used as part of a resonant circuit which is tuned to the resonant frequency of the MR device used. The resonant circuit is influenced according to the previously known process via an optical control signal which is supplied to the resonant circuit via an optical fiber. In the previously known process first in the usual manner in the entire examination volume of the MR device a high frequency excitation is performed by means of one or more high frequency pulses. The high frequency radiation couples into the microcoil so that the resonant circuit is excited to resonance. The excited resonant circuit then in turn emits a high frequency signal which in the local environment of the microcoil amplifies the field intensity of the high frequency pulse. By temporal variation of the optical control signal supplied to the resonant circuit, according to the previously known process the resonant circuit is switched on and off alternately by means of an optically controllable impedance. As a result the high frequency signal emitted by the microcoil also varies temporally according to the control signal. As a result the components of the received MR signal arising from the local environment of the microcoil can be distinguished from the signal components arising from the other areas of the examination volume. In this way it is possible according to the previously known process to identify the microcoil in the MR images reconstructed from the recorded MR signals and hence determine the position of the medical instrument.
The disadvantage in the previously known process is that it requires a special hardware, namely the opto-electronics described above, to control the resonant circuit. Furthermore special signal processing and signal generation components are required which must be controllable by the central control unit of the MR device. Examples are a suitable light source and a modulator to modulate the light of the light source which is coupled into the optical fiber of the medical instrument. It is necessary for the modulator to be controllable by the central control unit of the MR device so that the temporal development of the inputted light signal can be controlled in synchrony with the generation of the high frequency pulse and the recording of the MR signal. These components do not form part of the standard equipment of MR devices normally present in clinical use. Disadvantageously, to operate such devices according to the previously known MR processes, a significant investment is required to adapt the hardware and software.
On this basis the object of the invention is to develop an MR process which allows particularly simple location of a medical instrument equipped with a microcoil without the hardware of the MR device used requiring any special adaptation.
This object is achieved by the invention an MR process to locate in the examination volume of an MR device a medical instrument with attached microcoil which is part of a resonant circuit tuned to the resonant frequency of the MR device and having no external controls, where at least two temporally successive high frequency pulses are generated within the examination volume and where frequency-coded MR signals are recorded after each of the high frequency pulses from the examination volume. The position of the medical instrument is determined according to the invention by analyzing differences between the MR signals recorded.
It is known that in MR processes, generating two or more temporally successive high frequency pulses causes the core magnetization within the examination volume to try to reach a steady state. In the steady state, the generation of transverse core magnetization by the high frequency pulse and the relaxation of core magnetization are balanced. The steady state is usually only achieved after irradiation of several high frequency pulses. In the steady state the MR signals recorded have more or less constant amplitude. Until the steady state is reached the MR signal amplitude can be subject to oscillation, where in particular the intensity of these oscillations or the speed with which the steady state is achieved depends on the rotary angles of the core magnetization allocated to the high frequency pulses. As described above, excitation of the resonant circuit by the high frequency pulses leads to an amplification of the high frequency fields in the local environment of the microcoil attached to the medical instrument. As a result the effective rotary angle of the high frequency pulses in the local environment of the microcoil is greater than in the other areas of the examination volume of the MR device. The core magnetization to a certain extent “senses” a greater rotary angle of the high frequency pulse in the vicinity of the microcoil than in the areas remote from the microcoil. The process is based on the knowledge that the steady state of core magnetization in the areas of the examination volume remote from the microcoil is achieved in a different way to that in the immediate local environment of the microcoil. Frequency-coded MR signals from the examination volume are recorded after each of the high frequency pulses. This is suitably done by means of a magnetic field gradient in a prespecified spatial direction. Because the transition to the steady state in the local environment of the microcoil is different from that in the other areas of the examination volume, according to the invention the position of the medical instrument can be determined by simple analysis of differences between the MR signals recorded.
An advantage of the process is that with a microcoil on a medical instrument, only one resonant circuit need be applied, where the resonant circuit is firmly tuned to the resonant frequency of the MR device. The resonant circuit constitutes a purely passive high frequency circuit which has no any external controls. It is possible to fit to a medical instrument, for example an intravascular catheter, a suitable resonant circuit which in the simplest case comprises a microcoil and capacitor connected parallel thereto, with minimum expense and cost. It is particularly advantageous that no additional hardware components of the MR device used are required. The MR process according to the invention can simply be performed with any normal MR device in clinical use.
Suitably in the process a difference signal can be determined by subtraction of the recorded MR signals so that the position of the medical instrument can be determined from the frequency spectrum of the difference signal. The differences between the recorded MR signals can be analyzed particularly well by means of the difference signal. The position of the medical instrument arises on the basis of frequency coding of the MR signals recorded directly from the frequency spectrum of the difference signal. The frequency spectrum of the difference signal constitutes to some extent a projection image of the examination volume on the co-ordinate axis prespecified by the frequency coding direction. By use of this process, by recording the MR signals in different frequency coding directions, the precise three-dimensional position of the medical instrument within the examination volume can be determined. In particular the movement of the medical instrument within the examination volume can also be followed in real time. This process advantageously works extremely quickly as it is not necessary to record a complete MR signal data set to reconstruct an MR image of the entire examination volume. Advantageously this process can be used for “slice-tracking” where the position and orientation of the image plane for an MR layer image are prespecified as a function of the determined position of the medical instrument.
Suitably in the MR process, the rotary angles of the core magnetization allocated to the high frequency pulses are clearly less than 90°. It is advantageously possible to select the rotary angles allocated to the high frequency pulses such that the amplitudes of the successively recorded MR signal components which are based on the excitation of core magnetization in the local environment of the microcoil attached to the medical instrument show significant differences while at the same time the amplitudes of the MR signal components recorded from the other areas of the examination volume are essentially constant. Consequently by suitable choice of the rotary angle the determination of the position of the medical instrument can be optimized by ensuring that the amplitude of the core magnetization on transition to steady state in the local environment of the microcoil oscillates greatly while the amplitude of the core magnetization in the areas remote from the microcoil remains more or less the same. For the reliable function of the MR process however the rotary angles of the high frequency pulses are selected so that the transition of core magnetization to the steady state in the environment of the medical instrument differs from that in the other areas of the examination volume.
The temporal spacing between the successively generated high frequency pulses is less than the longitudinal relaxation time of the core magnetization.
To perform the MR process according to the invention an MR device implements the process described above by a suitable program control of the central control unit and/or reconstruction and display unit.
The process can be made available to the users of such MR devices in the form of a corresponding computer program. The computer program can be stored on suitable data carriers such as for example CD-ROM or diskette or it can be downloaded via the internet into the control unit of the MR device.